The present invention is generally directed to a process for the preparation of unsaturated polyester polymers, particularly useful for preparing low-fix temperature, crosslinked toner resins and processes thereof. In embodiments, the process comprises a two step monomer addition procedure of first transesterifying a diester, such as dimethyl terephthalate, and a glycol, such as 1,2-propanediol, followed by the subsequent addition of the unsaturated diacid, such as fumaric acid, to enable an unsaturated polyester with uniform distribution of unsaturated moiety on the polymeric chain useful for crosslinking to a higher gel, such as from about 0.1 percent to about 60 percent, with less initiator concentration, such as from about 0.05 to about 2 percent, and preferably from about 0.05 to about 1 percent by weight of resin. Toners containing the crosslinked polyesters obtained with the process of the present invention exhibit desirable low temperature fixing characteristics and excellent offset properties.
Conventional electrophotographic processes usually require temperatures of 160.degree.0 to 200.degree. C. to fix the dry toner, or toner particles on a support medium, such as a sheet of paper or transparency, creating a developed image. These high temperatures may reduce or minimize fuser roll life, such as fuser rolls of silicone rubbers or fluoroelastomers like VITON.RTM., may limit fixing speeds and may necessitate higher power usage during operation, such as in a xerographic copier employing a hot roll fixing mechanism.
Electrophotographic toners are generally prepared by mixing or dispersing a colorant and an optional charge enhancing additive into a thermoplastic binder resin, followed by micropulverization. Known conventional thermoplastic binder resins include polystyrenes, styrene acrylic resins, styrene methacrylic resins, polyesters, epoxy resins, acrylics, urethanes and copotymers thereof. Carbon black is often used as a colorant and alkyl pyridinium halides, distearyl dimethyl ammonium methyl sulfate, and the like can be employed as charge enhancing additives.
Although many processes exist for fixing toner to a support medium, hot roll fixing transfers heat very efficiently and is especially suited for high speed electrophotographic processes. In this method, a support medium carrying a toner image is transported between a heated fuser roll and a pressure roll with the image face contacting the fuser roll. Upon contact with the heated fuser roll, the toner melts and adheres to the support medium to fix an image.
Toner fixing performance may be characterized as a function of temperature. The lowest temperature at which the toner adheres to the support medium is referred to as the cold offset temperature. The maximum temperature at which the toner adheres to the fuser roll is referred to as the hot offset temperature. When the fuser temperature exceeds the hot offset temperature, some of the molten toner adheres to the fuser roll during fixing, is subsequently transferred to a substrate, a phenomenon known as offsetting, and results, for example, in blurred images. Between the cold offset temperature and hot offset temperature of the toner is the minimum fix temperature which is the minimum temperature at which acceptable adhesion of the toner to the support medium occurs. The difference between minimum fix temperature and hot offset temperature is referred to as the fusing latitude.
Several problems exist with the hot roll fixing system described above and with toners presently used with this system. For example, binder resins in the toners can require a relatively high temperature in order to be affixed to the support medium. A high temperature may result in high power consumption, low fixing speeds, reduced fuser roll, reduced roll bearing life, and offsetting.
Toner resins that have a low fix temperature below 200.degree. C. ("low melt toner resin"), preferably below 160.degree. C., and exhibit good offset temperature performance are desired. Processes for preparing such toner resins are, therefore, desirable. Toners operating at lower temperatures reduce power needs and increase component life. Low melt toners reduce volatilization of release oil such as silicone oil. Volatilization, which may occur during high temperature operation, causes problems when the volatilized oil condenses on other areas of the machine. Toners with wide fusing latitude permit liberal requirements for oil used as a release agent. The toners may provide improved particle elasticity and may minimize copy quality deterioration related to toner offset. Hence, the desirability of lowmelt temperature toner resins, particularly for use in hot-roll fixing xerographic processes, is apparent.
Resins with a lower minimum fix temperature usually possess a lower molecular weight than resins with higher minimum fix temperatures. U.S. Pat. No. 3,590,000 and U.S. Pat. No. 3,681,106 disclose the use of polyester resins as a toner binder. Although a minimum fix temperature of polyester binder resin can be lower than resins generated from other materials, such as styrene-acrylic and styrene-methacrylic resins, the use of polyester resin as toner binder can lead to undesirable lower hot offset temperature. This results in a decreased offset resistance, and a decreased glass transition temperature, possibly negatively impacting toner blocking, which occurs during storage.
Resin structure modification by branching, crosslinking, grafting, and the like using conventional polymerization and condensation reactions may also improve offset resistance. It is known that trivalent or more polyol or polyacid can be mixed with monomer to provide nonlinear polymer modification. Branching during polycondensation results in improved offset resistance. However, too much branching can result in an increased minimum fix temperature, diminishing any advantage of the modified polymer.
U.S. Pat. No. 4,533,614 discloses a nonlinearly modified lowmelting polyester containing: 1) an alkyl-substituted dicarboxylic acid and/or an alkyl-substituted diol; 2) a trivalent or more polycarboxylic acid and/or a trivalent or more polyol; 3) a dicarboxylic acid; and 4) an etherated diphenol. The main acid component of the polyester requires 50 mole percent, preferably 60 mole percent or more, of an aromatic dicarboxylic acid, its analogous anhydride, or other dicarboxylic acids to impart sufficient electrophotographic charge characteristics to a toner made from the resin. Modified polyesters having less than this amount of aromatic acid do not impart sufficient charge characteristics.
In U.S. Pat. No. 5,227,460, there is disclosed a commercially available poly(propoxylated bisphenol A co-fumarate) having a corresponding bisphenol residue in the polyester backbone. The linear resins disclosed in these references exhibit desirable rheological properties when crosslinked with, for example, benzoyl peroxide at a concentration of from about 0.01 to about 0.2 percent by weight, and enable a crosslinked polyester with from about 7 to about 40 percent gel content determined utilizing tetrahydrofuran as the solvent. The aforementioned linear unsaturated polyester resin can be prepared, for example, by condensing in a reactor a first monomer comprised of an organic diol with a second monomer comprised of an organic diacid such as fumaric acid with a catalyst. The corresponding product has an alternating structure of unsaturated maleate moiety with a saturated bisphenol A organic moiety.
In U.S. Pat. No. 5,348,830, the disclosure of which is totally incorporated herein by reference, there is illustrated a lower cost crosslinked polyester resin derived from the reaction of benzoyl peroxide with a lower cost unsaturated linear polyester, wherein the linear unsaturated polyester resin can be selected from about 50 mole percent of lower cost glycols, such as 1,2-propanediol and 1,2-ethanediol, and from about 30 to 45 mole percent of a low cost diester, such as dimethylterephthalate, and about 5 to about 20 mole percent of unsaturated diester or acid, such as fumarates. The process for the preparation of the aforementioned unsatured polyester is performed by condensing in a reactor a monomer comprised of an organic diol or mixture of diols with a monomer mixture comprised of an organic diester, such as dimethyl terephthalate and dimethylfumarate, with a catalyst. The monomers are thoroughly mixed to form a homogeneous solution. The solution is heated to a temperature at which a clear melt of the starting monomers is obtained, at which time a catalyst may be added to promote transesterification. Volatile alcohol byproducts from the transesterification, such as methanol or ethanol, is removed from the reaction system until more than 90 percent of the theoretical volatile alcohol has been removed. Transesterification catalysts may include tetraisopropyl orthotitanate transesterification catalyst, tetrabutyl orthotitanate monobutyl tin oxide, for example FASCAT.RTM. 4100, a registered trademark of M&T Chemicals Inc., and dibutyl tin oxide. This is then followed by further heating the transesterification product at a temperature ranging from about 150.degree. to about 250.degree. C., preferably from about 185.degree. C. to about 215.degree. C., and wherein the polycondensation reaction liberates the excess diol monomer which is removed under a generated vacuum, such as from about 760 millimeters-Hg to about 0.1 millimeter-Hg. Total reaction times may range from about 5 to about 16 hours. The resulting unsaturated polyester is then crosslinked in a melt mixer or alternatively by the reactive extrusion process as disclosed in U.S. Pat. No. 5,227,460, with the utility of benzoyl peroxide as the crosslinking initiator at a concentration of from about 0.25 to about 3.0 percent by weight. In Example X and Example XI of the '460 patent, about 1.7 percent by weight of benzoyl peroxide was utilized as the initiator for obtaining a crosslinked polyester based toner derived from low cost monomers such as 1,2-propanediol, 1,2-ethanediol, dimethylterephthalate and dimethylfumarate.
The use of excessive amounts of crosslinking initiator, such as benzoyl peroxide, is found to be undesirable because it tends to generate benzoic acid after it is consumed, thereby imparting high acid numbers in the toner composite. Toners with high acid numbers, such as from about 300 milliequivalents of potassium hydroxide or higher, are known to be undesirable for stable developments wherein the triboelectric charge of the toner developer is affected by environmental humidity conditions. Furthermore, the use of excessive amounts of crosslinking initiator, such as benzoyl peroxide, is undesirable during the manufacturing of the toner composites, especially during micronization or pulverization wherein benzoic acid particles as well as benzoyl peroxide particles are generated and cause contamination. Furthermore, the use of excessive peroxide initiator poses health risks to the workers. Additionally, lowering the amount or concentration of initiator, such as benzoyl peroxide, is of importance in obtaining a polyester with less impurities.
In the present invention, the use of the crosslinking initiator is minimized, such as from about 0.05 to about 1 percent by weight of unsaturated polyester, in yielding the lower cost crosslinked polyester based toner with low fixing and broad fusing latitude characteristics. The process of the present invention differs from the copending application U.S. Pat. No. 5,348,830 in that, for example, a two step addition, one pot procedure is utilized to assure that with the invention the unsaturated moiety is uniformly distributed throughout the polyester resin, hence permitting the use of lower amounts of crosslinking initiator, such as from about 0.05 to about 0.3 percent by weight to yield a crosslinked polyester based toner with low fixing and broad fusing latitude characteristics. More specifically, the process of this invention involves adding to a reactor with about 50 mole percent of diol or mixture of diols, such as 1,2-propanediol and 1,2-ethanediol, about 30 to 45 mole percent of low cost diester such as dimethylterephthalate and a transesterification catalyst, and by heating from about 160.degree. C. to about 185.degree. C. thereby generating and removing the transesterified product, such as methanol, in the distillation receiver until more than about 90 percent of the theoretical methanol is removed, followed by a second step wherein the unsaturated monomer, such as maleic anhydride, fumaric acid or dimethyl fumarate, is added to the reaction mixture, and further heating the transesterification product at a temperature ranging from about 150.degree. to 250.degree. C., and preferably from about 185.degree. C. to about 215.degree. C., and wherein the polycondensation reaction liberates the excess diol monomer, which can be removed under a vacuum, such as from about 760 millimeters-Hg to about 0.1 millimeter-Hg. Total reaction times may range from about 5 to about 12 hours. The unsaturated polyester derived from this two step addition process is found to crosslink with less initiator than a similar formulated polyester derived from the process of the prior art involving a one step addition as illustrated herein. It is believed that the present two step addition process, uniformly distributes the unsaturated moiety through the polyester chain, hence requiring less initiator concentration to generate similar gel contents.
In U.S. Pat. No. 5,391,452, the disclosure of which is totally incorporated herein by reference, there is illustrated a toner composition comprised of pigment, an optional charge enhancing additive and a side chain polymer comprised of a polyester present as a side chain on a polyalkylene of the following formula ##STR1## wherein m, n, and o represent the number of monomer segments present; R is independently selected from the group consisting of hydrogen and alkyl; R' is independently selected from the group consisting of aryl and alkyl; R" is independently selected from the group consisting of alkylene and oxyalkylene; and wherein m is from about 10 to about 1,000, n is from about 1 to about 1,000 and o is from about 10 to about 1,000. Also, in U.S. Pat. No. 5,376,494, the disclosure of which is totally incorporated herein by reference, there is illustrated a reactive melt mixing process of preparing low fix temperature toner resin, comprising the steps of (a) melting a reactive base resin, thereby forming a polymer melt; and (b) crosslinking said polymer melt under high shear to form a crosslinked toner resin.
There is a need for a crosslinked polyester derived from the crosslinking of an unsaturated polyester of low cost with minimal amount of crosslinking initiator. There is also a need for a crosslinked polyester based toner with low minimum fixing temperature, such as from about 120.degree. C. to about 145.degree. C., with broad fusing latitude of from about 30.degree. C. to about 60.degree. C., with low relative humidity sensitivity, such as from about 1 to about 2.3, as illustrated in copending application U.S. Ser. No. 251,430 (D/93410), the disclosure of which is totally incorporated herein by reference. These needs are achieved by the process of this invention wherein a two step addition method for the fabrication of the unsaturated polyester resin enables excellent uniformly distributed unsaturated moieties in the polyester chain, hence resulting in a higher degree of crosslinking with the utility of less crosslinking initiator.